Lithium batteries, in particular lithium secondary batteries, having such characteristics as a large energy density and a long life span, are used widely as power sources for home appliances such as video cameras and portable electronic devices such as notebook personal computers and mobile phones, electric tools such as power tools, and the like, and recently have been put into application in large batteries that equip an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like.
A lithium secondary battery is a secondary battery having a structure in which, during charging, lithium melts out from the positive electrode as an ion and moves towards the negative electrode to be stored and conversely, during discharging, the lithium ion returns from the negative electrode to the positive electrode, and it is known that the high energy density of the battery has its source in the electric potential of the positive electrode material.
In addition to lithium transition metal oxides such as LiCoO2, LiNiO2 and LiMnO2 having a layer structure, lithium transition metal oxides (in the present invention, referred to as “spinel type lithium transition metal oxides” or “LMOs”) having a spinel structure (Fd3-m) of the manganese series such as LiMn2O4 and LiNi0.5Mn1.5O4 are known as positive electrode active materials for lithium secondary batteries of this species.
Owing to low raw material costs and the absence of toxicity, which renders it safe, further more, having properties being strong against over-charging, there is a focus on the spinet type lithium transition metal oxide (LMO) of the manganese series as a next-generation positive electrode active material for a large battery for an electric vehicle (EV), a hybrid electric vehicle (HEV) and the like. In addition, a spinel type lithium transition metal oxide (LMO), which allows for insertion and desorption of Li ions three-dimensionally, has excellent output characteristics, compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, such that utilization in applications requiring excellent output characteristics such as tools called power tools, EV and HEV batteries and the like, are anticipated, and additional improvement of output characteristics are intended.
As a spinet type lithium transition metal oxide (LMO) with improved output characteristics, in prior art, a lithium manganese composite oxide represented by the composition formula Li1+xMn2−xOu−yFy(where 0.02≦x, 0.1≦y≦u, 3≦(2u−y−1−x)/(2−x)≦4 and 3.9≦u≦4.1) having a mean particle diameter in the range of 1 to 20 μm is described in Patent Reference 1.
In addition, an Li—Mn series spinel compound represented by the composition formula Li1+xMn2−x−yMgyO4 (x=0.03 to 0.15, y=0.005 to 0.05) having a specific surface area of 0.5 to 0.8 m2/g and a sodium content of 1000 ppm or less is described in Patent Reference 2.
Since a spinel type lithium transition metal oxide (LMO) has a small filling density compared to a lithium transition metal oxide such as LiCoO2, which has a layer structure, it has the problem that the discharge capacity per volume is low.
Therefore, for instance, Patent Document 3, or the like, describes a technique, which, by adding an element of an oxide which melting point is 800° C. or lower, for instance boron (B) and firing, promotes the generation and growth of crystals of spinel type lithium transition metal oxide (LMO), that is to say, promotes sintering of micro-particles which are assembled crystal particles, and elevates the filling density (tap density) as compact micro-particles.